Apparatus and process for induction heating



y 0, .1968 A. F. LEATHERMAN ETAL 3,395,261

APPARATUS AND PROCESS FOR INDUCTION HEATING Filed on. 19, 1965 United States Patent 3,395,261 APPARATUS AND PROCESS FOR INDUCTIUN HEATING Alfred F. Leather-man, Upper Arlington, Columbus, Ohio,

and William C. Heller, Jr., 1840 N. Farwell Ave., Milwaukee, Wis. 53262; said .Leatherman assignor, by mesne assignments, to said Heller Filed Oct. 19, 1965, Ser. No. 497,705 13 Claims. (Cl. 21910.61)

ABSTRACT OF THE DISCLUSURE An induction heating apparatus includes a pair of closely spaced conductors connected to a power source. Each of the conductors includes an axle portion on which is mounted an electrically conductive pressure wheel. The pressure wheels have a small clearance between them. The apparatus may be utilized in a process comprising the steps of applying a susceptor to a non-metallic material to be treated by heating and pressure, energizing the apparatus to provide a magnetic field, and passing the material and the susceptor through the clearance between the pressure wheels to heat the susceptor and the material while simultaneously applying pressure.

This invention relates to induction heating apparatus and processes and more particularly to an apparatus and process having improved operating efficiencies and which permits the application of pressure to the material being inductively heated.

Induction heating is a thermal process in which, in its well known forms, electrical energy in the form of a high intensity, high frequency magnetic field is applied to a metallic substance. The field induces eddy currents or hysteresis losses, which cause heat to be generated in the substance itself. This method has been in common use for melting and heat treating metals for a number of years.

Induction heating may also be used in the thermal processing of non-metallic materials, such as plastics, by placing inductively heatable substances, as for example, certain metal or metal oxide structures or particles, at points in the material where heat is desired, and then placing the composite structure in a magnetic field. For example, if it is desired to join two sheets of plastic, such as polyethylene, fine metal or metal oxide particles, or a metal screen may be placed between the sheets at the points desired to be joined. When a magnetic field is applied to the sheets, the particles or screen become heated, softening the plastic and allowing the two sheets to fuse. Pressure may then be applied to the sheets to insure a firm bond between them. The metallic particles or structure is generally termed a susceptor to indicate its capability of being heated by a magnetic field.

The above method of thermal processing differs from dielectric thermal processing in which a non-metallic substance is itself heated by a high frequency electric field. Dielectric thermal processing involves considerations not here pertinent.

The advantages of induction thermal processing include the fact that heat is generated only at the location where it is to be used, thereby providing ideal temperature distributions and permitting accurate and beneficial control of temperature. Additionally, since heat is not required to flow from an external source through the materials to the required location, substantial increases in the rate of thermal processing are obtainable. The accurate temperature control and shortened exposure times prevent thermal damage, such as charring, warping, or distortion from occurring during the processing.

3,395,261 Patented July 30, 1968 ice It is necessary, in order to obtain the above advantages in a commercially and technically feasible process, such as the heat sealing of plastics, to provide an induction heating apparatus capable of producing a magnetic force, or field of the highest possible intensity and of the highest possible frequency in order to generate the largest amount of heat by induction processes. The equipment used to generate such a field generally consists of a field producing apparatus (e.g., work coil) coupled to a high frequency power source. The attainment of both the aforementioned criteria depends to a great extent upon reducing the inductance of the apparatus to the lowest possible value.

An excessive amount of inductance in the apparatus limits the magnitude of the high frequency current fiowing through the apparatus and hence the intensity of the magnetic field generated thereby. While a greater applied voltage may be used to increase current flow, this can result in inefiicient operation of the field producing apparatus. The size, or configuration, of the field producing apparatus may also be reduced or changed to lower the inductance but often only at the expense of a decrease in processing speed or capacity of the equipment.

Further, it is generally necessary to operate the apparatus in parallel resonance with the power source as the current fiow at such a frequency is maximized. Resonant frequency is determined by the formula A low value of inductance permits the resonant frequency of the apparatus to be high enough to generate induction losses of the required magnitude. For example, a resonant frequency of 4 megacycles may be required in thermal processing non-metallic materials. This is significantly above the frequencies required for metallurgical uses which generally range from 3 kilocycles to 450 kilocycles.

'It is also desirable to provide an induction heating apparatus which concentrates the magnetic field in the substance undergoing thermal processing since portions of the magnetic field not in the substance serve no useful purpose while increasing the inductance of the apparatus.

Many thermal processes require the application of pressure to the substance being thermally processed. While the need is very apparent in the processing of sheets, or other web materials, as for example during bonding operations, it also exists in many of the other applications of induction heating.

In the prior art the means for applying pressure to the thermally processed substance has often been separate from the induction heating apparatus since the addition of the pressure means increased the inductance of such apparatus to the extent that proper thermal processing could not take place. Generally, the pressure means was placed behind the induction heating apparatus so that the material was first heated and then compressed. Such an arrangement permitted the material to cool down in the space between the heating apparatus and the pressure means. In many applications this temperature drop was of considerable magnitude thereby affecting the quality of the finished product. Additionally, separate heating apparatus and pressure means increased the expense and complexity of the thermal processing equipment.

Other prior art equipment has utilized field producing apparatus having pressure wheels integrally mounted on the current carrying portions thereof to apply pressure to a Prior art apparatus that has not used integrally mounted pressure wheels to compress the substance being thermally processed have generally slid the substance across the induction heating apparatus and utilized the friction or web tension generated thereby to compress the substance. See, for example, the structure described in application Ser. No. 374,470 filed June 11, 1964 and assigned to the same assignee. However, since the substance was often in a softened state due to the inductive heating it was very prone to stick or tear while sliding across the induction heating apparatus.

It is, therefore, an object of this invention to provide an induction heating apparatus of low inductance which utilizes such low inductance characteristics to produce a high intensity, high frequency, magnetic field suitable for efficient induction heating.

Another important object of this invention is to provide an induction heating apparatus having a minimal value of inductance which may be used to apply pressure to the material being inductively thermal processed.

A further object of this invention is to provide a low inductance induction heating apparatus which concentrates the high intensity, high frequency magnetic field in the material being inductively heated so as to permit efficient utilization of the apparatus for induction thermal processing.

Yet another object of this invention is to provide an induction heating apparatus having a minimal value of inductance so as to provide a superior electrical circuit for the apparatus power supply.

A further object of this invention is to provide an induction heating apparatus meeting the above objects, which, through its use, permits substantial increases in the rate of thermal processing and which prevents sticking or tearing of the thermally processed substance.

Yet another object of this invention is to provide an induction heating apparatus which is simple in construction and manufacture, thereby providing substantially troublefree operation for substantial periods of time.

It is another object of this invention to provide an improved process of inductively heating non-metallic materials which process utilizes apparatus having the fore going features to obtain accurate temperature control at high processing speeds without the risk of thermal damage to the material while simultaneously applying pressure to the material being inductively heated, such process being particularly adapted for use in sealing or uniting non-metallic web materials.

Briefly, the invention provides a pair of closely spaced conductors forming a magnetizing loop. In their center, the conductors are spread apart and serve as axles for metal pressure wheels mounted thereon. These wheels apply pressure during the thermal processing of material placed between them. Low inductance for the apparatus is obtained by keeping the conductors as closely spaced as possible except for the portions serving as axles for the pressure wheels and by designing the conductors and wheels to utilize the skin effect and proximity effect of the currents flowing therethrough to insure the current passing along adjacent areas of the pressure wheels. Not only does this decrease the inductance by decreasing the area encompassed by the effective current conductors, it also concentrates a magnetic field in the substance passing between the pressure wheels. In the alternative embodiment of the invention a pair of flanges are mounted on the conductors adjacent the pressure wheels. The flanges further utilize proximity effect of conductors to reduce the inductance of the induction heating apparatus.

In the thermal process of the present invention, as for example sealing a plurality of webs of plastic having a susceptor between them, the webs are passed between the pressure wheels. This exposes the susceptor to the magnetic field created by the induction heating apparatus, inductively heating the susceptor and, by conduction therefrom, heating the webs. At the same time the pressure wheels on the conductors apply pressure to the plastic insuring a highly satisfactory seal between the layers.

The invention, together with its features and mode of operation, may be better understood by reference to the following specification and drawings forming a part thereof, in which:

FIGURE 1 is a perspective view of one embodiment of the present invention;

FIGURE 2 is a front view of the portion of the embodiment of FIGURE 1 encompassed by the brackets 2-2, the figure serving to illustrate the process of the present invention;

FIGURE 3 is a cross sectional view of a modification of the embodiment of the invention shown in FIG- URE 1;

FIGURE 4 is a perspective view of another embodiment of the present invention; and

FIGURE 5 is a front view of a further modification of the present invention.

Referring to the figures, and particularly FIGURES 1 and 2, there is shown therein one embodiment of the induction heating apparatus 6 of the present invention. The apparatus includes a pair of closely spaced conductors 8 and 10. The conductors, which may be constructed of hollow copper tubing are arranged in a narrow loop having initial portions 12 and I4 and must have a common junction portion 16. Conductors 8 and 10 have initial portions 12 and 14 connected to high frequency source 18 which, by way of example, may provide conductors 8 and '10 with alternating current of a frequency of 4 megacycles per second. Initial portions 12 and 14 may also be connected to a coolant supply 20 which circulates a coolant, such as water, through the tubing comprising conductors -8 and Ill.

Inthe middle portion thereof, conductors 8 and 10 are spread apart to serve as axles for pressure wheels 22 and 24. Specifically, conductor 8 includes portions 7 and 9 at right angles to the initial portion 12 and loop portion 16 and axle portion 11 parallel to the latter portions. Similarly, conductor 10 includes portions 13 and 15 and axle portion 17. Portions 7 and 9 and 13 and 15 closely abut the opposing adjacent faces of pressure wheels 22 and 24.

Pressure wheels 22 and 24, which may also be constructed of copper, contain center holes 26 and 28 which mount the wheels in opposing relation on axis portions 11 and 17. A small clearance 30 exists between the pressure wheels.

The material 31 to be thermally processed passes through clearance 30 under pressure from wheels 22 and 24. Additional pressure may be supplied to the material by backup rolls 32 and 34. These rolls, constructed of rubber or other insulating material, compressively bear on pressure wheels 22 and 24.

The electrical characteristics of the above structure are those of low inductance for the following reasons. The space between conductors 8 and 10 is kept to a minimum as is a spacing between portions 7 and 9 and wheel 22 and portions 13 and 15 and wheel 24. These closely spaced conductors have low inductance since current flow in one such portion or wheel tends to cause current flow in the opposite direction in the adjacent portion or wheel, thus aiding the natural fiow of current in the circuit. Further, a smaller enclosed coil area offers a high reluctance to magnetic field lines, meaning that a smaller total magnetic field, and hence lower inductance, will exist for a coil enclosing a smaller area. The closely spaced portions and wheels of the above structure therefore have a considerably lower inductance than do the prior art structures wherein the members were spaced more widely apart.

Secondly, the features of the present structure may be attributed to the so-called skin effect which causes high frequency current to be transmitted along the surface of the conductor. In essence, this effect is caused by the greater back E.M.F. introduced in the central portions O of the conductor due to the greater amount of flux surrounding those portions than the outer portions. The central portion of a conductor, therefore, may be considered to have a higher inductive reactance than the outer portions. The high frequency current naturally tends to follow the path of lowest inductive reactance and hence travels along the outer surface of a conductor. Further, the path that the high frequency current will follow is the path of least enclosed area since this area, for the reasons described in the preceding paragraph, is also a path of lower inductive reactance than other alternative paths in the circuit.

The path of the high frequency current from high frequency source 18 through inductive heating apparatus 6 is shown by the arrows in FIGURE 2 for an instantaneous situation. Specifically, the current is shown as entering the initial portion 14 of conductor and flowing through portion to pressure wheel 24. As described above, the high frequency current tends to follow the path of smallest enclosed area and thus flows down pressure Wheel 24 towards clearance 30, across pressure Wheel 24 and back up the other side to portion 13. The path of the current toward and away from clearance parallels portions 15 and 13 because of the proximity effect of the flowing currents which causes them to flow in parallel paths. The current then proceeds around junction portion 16 to portion 7 and thence to pressure wheel 22 where its path is similar to that of the current path in pressure wheel 24. The current is then returned to high frequency source 18 through portion 9 and initial portion 12 of conductor 8. The flow of the current in both pressure wheels along clearance 30* concentrates the current, and hence the magnetic field in the clearance, thereby providing an efiicient induction heating apparatus. For all intents and purposes, the current may be considered to proceed in a horizontal direction axially along either side of clearance 30 from initial portion 14 across the inner portion of pressure wheel 24, around junction portion 16, across the inner portion of pressure wheel 22 to initial portion 12. Reversal of the alternating current from high frequency source 18 reverses the direction of current flow through induction heating apparatus 6 In operation, the substance to be inductively thermal processed is placed in clearance 30. As previously mentioned, induction heating apparatus 6 is ideally adapted for use with a web material and for exemplary purposes a web of plastic material in which it is desired to make a lap seam is shown in the figures. Specifically, the web comprises two portions 36 and 38 having a layer of iron oxide particles between them. The sheets along with layer 40 are placed between pressure wheels 22 and 24 and the inductive heating apparatus energized by high frequency source 18. The magnetic field generated in clearance 30 by induction heating apparatus 6, in the above described manner, inductively heats iron oxide layer 40, softening the adjacent surfaces of plastic portions 36 and 38. Pressure wheels 22 and 24 apply pressure to portions 36 and 38 thereby securing a satisfactory bond between the overlapping portions of the web. If additional pressure is desired, back up rolls 32 and 34 may be utilized. As the web is moved past pressure wheels 22 and 24 to expose additional portions of the Web to the magnetic field in clearance 30', the pressure wheels rotate to provide frictionless passage of the web between the wheels. This prevents the sticking or tearing experienced with some prior art structures. Depending on the substance being thermally processed, it may be desirable to coat the surface of pressure rolls 22 and 24 with a low friction coating 25 such as tetrafluoroethylene to further facilitate the passage of the substance past pressure wheels 22 and 24. See FIGURE 3. This also prevents short circuits, should pressure wheels 22 and 24 accidently come in contact with each other.

Even though pressure wheels 22 and 24 rotate in response to the passage of the web, current from power source 18 still flows along the circumference of the wheels adjacent clearance 30 as this is the path of least enclosed area.

FIGURE 3 shows an embodiment of the invention similar to that shown in FIGURES 1 and 2 and described above, wherein pressure rolls 22 and 24 are elongated to inductively thermal process a wider portion of the web. The electrical principles of such a structure are identical to those described in connection With FIGURES l and 2.

FIGURE 4 shows an embodiment of the invention shown in FIGURE 2 in which it is not necessary to thread the Web between initial portions 12 and 14 of conductors 8 and 10 and through common portion 16. Specifically, the latter portion includes a section 42 which runs parallel to the web and a section 44 running across the web. Similar portions 46 and 48 are positioned beneath the web so that the common portion 16 lies to one side thereof. Portions 44 and 46 may be constructed to be broader in design in the direction parallel to Web travel so as to cause the current in these conductors to spread out as it passes over the web. This spreading would reduce any tendency for reheating to occur when web travel is to the right in FIGURE 4. Conversely, conductors 44 and 46 essentially in their present form will serve to supply preheating in the case for which Web travel is from right to left.

Alternatively, portions 44 and 46 may be utilized to support a second set of pressure wheels thereby providing for heating of the web in two steps and permitting a lower output power setting to be used for high-frequency source 18, or a faster processing speed with the same power setting.

FIGURE 5 shows an alternate embodiment of the invention described above. This structure is similar to that shown in FIGURES 1 and 2 with the addition of copper flanges 50 and 52 mounted on portions 9 and 7 of conductor 8 adjacent pressure wheel 22 and flanges 54 and 56 mounted on portions 15 and 13 of conductor 10 adjacent pressure wheel 24. In addition to positioning pressure wheels 22 and 24 on axle portions 11 and 17, these flanges serve to further reduce the inductance of induction heating apparatus 6 by utilizing the proximity effect of flowing currents. This effect involves conductors carrying high frequency current in close proximity such as flanges 50 through 56 and pressure wheels 22 and 24. Without the flanges the flux density in the confined spaces 58 through 64 is higher than elsewhere in the magnetic circuit generated by the current from high frequency source 18. The result, without flanges 50 through 56, would produce excessive rates of heating in these areas.

The flanges 50 through 56 may, by way of example, be approximately the same diameter as pressure wheels 22 and 24 and may be spaced .001" to .05" from the ends thereof.

By virtue of the proximity effect, one current path tends to become a mirror image of a neighboring current path. Thus the current path in pressure wheels 22 and 24 becomes a circular mirror image of flanges 50 through 56. With the Wider face provided on conductors 8 and 10 by flanges 50 through 56 the high density current in the flanges and in pressure wheels 22 and 24 tends to spread out. The greater area of these current paths reduces the power losses experienced in this portion of the circuit and hence increases the overall efliciency of the induction heating apparatus 6. Without the flanges 50 through 56. the current path on the side of pressure wheels 22 and 24 would tend to concentrate in the narrow image of portions 7, 9, 13 and 15 of conductors 8 and 10. This causes a concentrated area of power loss that can become of significant magnitude.

It is noted that the side by side current paths in the flanges 50 through 56 and in the pressure wheels 22 and 24 are electrically connected in parallel, further reducing the inductance of these paths. Also, as the currents divide into parallel paths, the concentration of current is reduced, significantly spreading the power losses and producing a lower total loss than before since local power loss is proportional to the square of the current at any oint.

p The employment of the embodiment shown in FIG- URE is similar to that shown in FIGURES 1 and 2. Further, the modifications shown in FIGURES 3 and 4 are equally applicable to the embodiment shown in FIG- URE 5.

Other modifications and alterations are contemplated and it is desired to include all such modifications and alterations as come within the true scope and spirit of the following claims.

We claim:

1. A low inductance induction heating apparatus for generating a high frequency, high intensity magnetic field from a high frequency power source including integral means for applying pressure to a substance being inductively heated comprising:

a pair of electrically connected, closely spaced conductors connected to said power source, each conductor including upstanding portions supporting a spaced axle portion; and

an electrically conductive pressure wheel mounted on each of said axle portions with a small clearance between the wheels, said wheels being closely spaced from said upstanding portions, and being connected to said conductors to form an integral current carrying portions of said apparatus;

whereby said pressure wheels apply pressure to said substance as it moves through said clearance and the close spacing of said conductors and said portions and wheels reduce the inductance of said heating apparatus while providing a concentrated magnetic field in said clearance.

2. The induction heating apparatus of claim 1 wherein said upstanding portions have electrically conducting flanges mounted thereon adjacent said pressure wheels, said flanges serving to further reduce the inductance of said induction heating apparatus.

3. The induction heating apparatus of claim 1 including a pressure roll compressively abutting said pressure wheels to supply further pressure to the material passing through said clearance.

4. The induction heating apparatus of claim 1 wherein said pressure wheels are coated on their cylindrical surfaces with an insulating coating.

5. The induction heating apparatus of claim 4 wherein said insulating coating is tetrafiuoroethylene.

6. The induction heating apparatus of claim 1 wherein said closely spaced conductors are bent to one side of said pressure wheels and out of alignment therewith to permit the substance being inductively heated to be inserted in the clearance between said pressure wheels without being threaded through said closely spaced conductors.

7. The apparatus of claim 2 wherein said flanges are of approximately the same diameter as said pressure wheels.

8. The apparatus of claim 2 wherein said flanges are spaced .001" to .05" from said pressure wheels.

9. A process for inductively heating a non-metallic material while simultaneously applying pressure thereto comprising the steps of:

applying a susceptor to a portion of a non-metallic material to be treated by heating and pressure;

providing a high intensity, high frequency magnetic field by means of an induction heating apparatus comprised of a pair of electrically connected, closely spaced conductors, each conductor including upstanding portions supporting a spaced axle portion, and an electrically conductive pressure wheel mounted on each of said axle portions with a small clearance between the wheels, said wheels being closely spaced from said upstanding portions and being connected to said conductors to form integral current carrying portions of the apparatus; and

pass-ing said material with said susceptor through the small clearance between said pressure wheels, to expose said susceptor to said high intensity, high frequency magnetic field and to cause said susceptor to become heated, so as to heat said material by conduction while said pressure wheels simultaneously apply pressure thereto.

10. The process of claim 9 wherein the induction heating apparatus includes electrically conductive flanges mounted on the upstanding portions of said conductors adjacent said pressure wheels, said flanges serving to increase the intensity of said magnetic field.

11. The process of claim 9 wherein said material is a plurality of webs of plastic to be sealed together and the susceptor comprises a layer of iron oxide particles between the webs.

12. The induction heating apparatus of claim 1 wherein said closely spaced conductors are bent to one side of said pressure wheels and out of alignment therewith to form conductors parallel to said axle portions, said conductors including upstanding portions supporting second spaced axle portions having electrically conductive pressure wheels mounted thereon so as to form a small clearance between the wheels, said wheels being closely spaced from said upstanding portions and being connected to said conductors to form integral current carrying portions of said apparatus, whereby the substance being inductively heated may be inserted in the clearance between said pressure wheels without being threaded through said closely spaced conductors.

13. The induction heating apparatus of claim 12 wherein said upstanding portions have electrically conductive flanges mounted thereon adjacent said pressure wheels, said flanges serving to further reduce the inductance of said induction heating apparatus.

References Cited UNITED STATES PATENTS 2,299,934 10/1942 Sherman et al. 2l9l0.79 X 2,761,941 9/1956 Ardichivli 219-l0.49 3,322,928 5/1967 Pungs et al. 2l9l0.79 X

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner. 

